1. Field of the Invention
The present invention relates to a display, and more particularly, to an organic light emitting diode (OLED) display using thin film transistors.
2. Description of the Related Art
In recent years, the displays using an organic light emitting diode which consists of thin organic films with electroluminescence (EL) in each pixel receive attention as a next generation flat panel display to replace a liquid crystal display. The display, that is, an organic light emitting diode display referred to as OLED display, is different from the conventional liquid crystal display in that, because of the self-emission phenomenon, an additional light source is not necessary, which enables decrease in volume and in weight of the display. Further, the organic light emitting diode display has advantages of low power consumption, a wide angle of view, high brightness, fast response, and the like over the liquid crystal display.
Because of these characteristics, the organic light emitting diode display is placed as a mighty next generation display to be used in household electronic appliances such as a cellular telephone and a flat TV set, and is now energetically developed.
Drive systems of the organic light emitting diode display are roughly classified into a passive matrix type and an active matrix type. In a passive matrix organic light emitting diode display, bias voltage is applied to a signal electrode and a scanning electrode at the same time, and an organic layer between the electrodes is caused to emit light. Although the structure is simple and the cost of manufacturing may be suppressed, in a trend toward a larger area display, there are also disadvantages such as relatively high power consumption, low resolution, slow response, and a lowered aperture ratio in accordance with an increased number of wirings. An active matrix organic light emitting diode display has advantages of lower power consumption, higher image quality, faster response, higher light emission efficiency, and the like compared with the passive matrix organic light emitting diode display. Therefore, the active matrix type is regarded as a preferred drive system of a large area organic light emitting diode display.
A pixel circuit of the active matrix organic light emitting diode display includes at least an organic light emitting diode and a plurality of thin film transistors. As the above-mentioned thin film transistors, at least a switch transistor for controlling the brightness of the pixel and a drive transistor for controlling light emission of the organic light emitting diode are necessary.
Here, performance characteristics of the active matrix organic light emitting diode display are briefly described with reference to a drawing. FIG. 14 illustrates an exemplary structure of one basic pixel circuit of the active matrix organic light emitting diode display. As illustrated in FIG. 14, a selection line 106 and a data line 105 which is orthogonal to the selection line 106 are formed on a substrate. A switch transistor 111 is placed at a point at which the data line 105 and the selection line 106 intersect each other. Part of wiring of the switch transistor 111 is connected to a drive transistor 112. A power line 107, a storage capacitor 110, and an organic light emitting diode 109 are also connected to the drive transistor 112. First, a gate signal is applied from the selection line 106 to the switch transistor 111. A current signal flowing through the data line 105 becomes a voltage signal through the switch transistor 111 and is applied to a gate electrode of the drive transistor 112. This causes the drive transistor 112 to operate, a current is allowed to flow through the organic light emitting diode 109, and the organic light emitting diode 109 emits light. Here, the storage capacitor 110 has a function of retaining the signal at the gate electrode. More specifically, the current flowing through the organic light emitting diode 109 is adjusted to be at a fixed level until the next signal is applied even after the switch transistor 111 is turned off.
As described above, because the organic light emitting diode display is a current-driven display, the respective thin film transistors constituting the pixel circuit are required to have the following characteristics. The switch transistor is required to sufficiently suppress an OFF current in order to suppress fluctuations in brightness. The drive transistor is required to have high carrier mobility and a threshold voltage which does not shift in order to flow a enough current that the organic light emitting diode is able to emit light and in order to maintain the current-carrying state. Further, because the drive transistor uses a saturated region of output characteristics, the drive transistor is required to have satisfactory saturation characteristics in which the current value is constant in the saturated region.
Japanese Patent Application Laid-open No. 2008-124392 discloses a semiconductor device including a semiconductor layer with a laminated structure of a low-crystallinity semiconductor layer containing both an amorphous phase and a crystal phase and a high-crystallinity semiconductor layer having a higher crystallization rate than that of the low-crystallinity semiconductor layer, which are formed in an order from the side of a contact layer.